Well, since they have to have the same number of hours and one of them gets shifts in blocks of 12 hours, the minimum needed would be 12 hours.
one 12-hour shift is the minimum and one 12-hour shift is the same time as six 2-hour shifts.
Roxanne will need to keep that pattern (one 12-hour shift and six 2-hour shifts) to keep the hours the same. So she could assign 12 hours, 24 hours, 36 hours, etc.